torsional vibrations in turbomachinery - laborelec
TRANSCRIPT
Torsional vibrations in turbomachinery
Should I worry about it? How can I manage the risk?
Q & A
Webinar
20 October
2020
2
Does the effect of DC conversion stations on torsional vibration vary depending on whether you’re exporting power from your grid or importing it?
There are different situations where a converter
station may cause torsional vibrations at a nearby
power plant. For the Line Commutator Converter (LCC)
type, interaction is only considered to be possible at
the rectifier side, so when exporting power from the
AC grid to the DC line. Other issues have recently been
reported in China, where a large number of converter
stations are being installed. There, issues are also
reported on the inverter side (importing power to AC
grid) after numerous commutation failures.
What is the relation between electrical sub-synchronous frequency (sub-harmonics) and mechanical torsional frequency of the shaft?
The torsional frequency fm seen on the shaft is equal
to the grid frequency f0 minus the electrical frequency
fe. So fm=f0- fe. As the shaft is running at a certain
speed, the electrical frequency becomes modulated on
top of this rotational speed. So, a 40Hz component on
the grid will be seen as 10Hz mechanically. A torsional
frequency fm will induce electrical components at
fe=f0 +/- fm, so both sub-and supersynchronous
components.
Concerning electrical frequency and mechanical torsional frequency, can one measurement determine the other?
Yes, but reconverting the electrical components
into torsional vibration amplitudes is far less
straightforward.
What is a safe permissible torsional vibration level for GT & Generator?
The underlying damage mechanism for torsional
vibrations is fatigue. For every torsional vibration
mode, the most stressed location on the shaft can be
different and hence the allowable torsional vibration
level at the GT/Generator can be different too. It
also depends on the local geometry, local stress
concentration effects, material, and mean stress, for
example. You also need to consider the measurement
position. So, there is no generally applicable rule.
Is the second mode of 95Hz typical for the given example of a turbo machine, and is it different for each shaft line? What is the determining factor for this 95Hz frequency?
Every dynamic structure is characterized by its
natural frequencies, which in turn are determined by
the inertia and stiffness properties of the shaft line.
For the gas turbine unit in the example, the three
lowest natural frequencies are 18Hz, 95Hz and 130Hz.
They will be different for other gas turbine units, but
qualitatively it will be comparable.
Typically, there is one subsynchronous natural
frequency, around 15-25Hz, at which the gas turbine
twists out-of-phase with the generator. Higher modes
involve twisting of the main rotors (gas turbine
against compressor, internal twist at generator) and
are therefore higher. For ST units or single shaft units,
it is more complex and multiple subsynchronous
natural frequencies are seen.
Q & A Webinar 20 October 2020
Frequently asked questions about torsional vibrations in turbomachinery
3
Severe compressor rotor cracking caused by torsional
vibration resulting in losses of several millions of Euros.
This huge crack in a compressor rotor shaft was recently discovered
at a power plant. The location and orientation of the crack suggests
that it was caused by torsional vibrations. For the power plant, this
came as a complete surprise. They were under the impression that
their shaft line was running smoothly, while in fact excessive torsional
vibrations had been present for almost 10 years, but went undetected
because they occurred only in very specific conditions. Because the
cracking lay undiscovered for so long, fatigue damage accumulated
and finally led to enormous financial losses.
You speak about CCGT turbines. Are these phenomena applicable to hydraulic turbine shafts?
Torsional interaction can occur in every unit, so steam
turbine units too, for instance. For hydro turbines,
the risk of grid interaction is typically low because
the generator has a very large inertia compared to
the turbine. It therefore acts as a filter, effectively
blocking oscillating energy from the grid. The water
from the turbine also provides more damping to the
torsional vibrations compared to GT and ST units
where the overall damping is very low (only material
damping and small damping due to steam at larger LP
blades).
Less stable grid: what would be the shortest (or longest) phenomenon that could trigger a harmful torsional vibration? Can normal changes in wind speed or direction, or variations in sun cause damage?
Normal changes in wind or solar production have a
negligible effect on torsional vibrations. This holds for
transient events in general except, for example, in the
case of short circuits at the generator clamps of the
power plant. As long as the transient events involve
power changes of less than 50% of the nominal power
of the power plant, they are not considered lifetime
consuming.
Renewables, however, render the grid less stable
as grid inertia reduces. A stronger rate of change of
frequency is therefore already considered (for Europe,
see available ENTSO-e guidance documents). A less
stable grid can also cause more contingencies (e.g.
loss of a transmission line), impacting the potential
interaction of the power plant with the grid. Other
changes include more and larger power electronics on
the grid which can also increase the risk of interaction.
Does the added mass/inertia/damping at turbine impeller vanes influence torsional vibrations?
For a turbine impeller, the inertia of the water must
be taken into account as it lowers the torsional
natural frequency of the system. Typically, the water
is added as a lumped mass. Hydro-acoustic effects
are typically not considered for the overall torsional
vibration behaviour of the shaft line. They do need
to be considered when the dynamics of the impeller
itself are analysed. The water also introduces some
damping in the system which reduces the torsional
vibration amplitudes.
In 2006, a major grid incident occurred across Europe with under- and over-frequency of +/- 1.5 Hz. Was there any known damage caused by this incident?
We still have the grid frequency measurements
for Belgium from that time. A change in the grid
frequency is not necessarily an issue for the shaft line
as long as this change is limited to avoid excitation
of torsional natural frequencies nearby 1x and 2x
grid frequency. The incident could have been more
significant in terms of changes in grid configuration
and grid strength which in turn increases the risk of
torsional interaction.
We are not aware of any damage from this incident
but, since it concerns fatigue damage, it cannot be
excluded that rotor lifetime was consumed at some
power plants.
What exactly is the principle behind the measurement Laborelec makes at the end of the generator shaft? Is it mechanical, or electrical?
We have implemented an optical encoder at the shaft
end of our nuclear units. The advantage is that you
have a perfectly uniform toothed wheel (for fewer
harmonics) and also a 1 per revolution signal which
facilitates post-processing.
4
Our insurer allows for measuring just one of a series of similar shaft lines. Would a different location be a contra-indication for this approach?
That depends on what you are aiming for. If the goal is
to confirm 1x and 2x grid frequency exclusion zones
for the torsional natural frequencies of the shaft line,
then this seems justified. If potential interaction with
the grid is under study, additional analysis would be
required.
Are torsional vibrations specific to particular types of gas or steam turbines?
When issues are caused by grid interaction, this is
not fleet specific. The interaction can happen for
every type of machine. Identical machines are not
necessarily subject to the same interaction since the
local grid configuration plays a role here.
If the underlying cause is internal to the power
plant, for instance due to voltage or speed control
instabilities, this can be a fleet issue and therefore
fleet/machine specific.
This is exemplified by two recent examples in the
MHPS fleet after which a technical bulletin was
distributed to the entire MHPS fleet.
How would you compare the magnitude of torsional vibrations induced by a generator trip with a load, versus all other causes (such as grid variations)?
A trip of the unit at nominal load will cause a torsional
oscillation of the lowest torsional natural frequencies
with an amplitude initially equal to the static angle at
nominal load. In terms of fatigue lifetime consumption,
a trip event is considered to have very limited impact.
Grid interaction can induce much higher torques (e.g. 5x
static torque) which can also last longer.
Q & A Webinar 20 October 2020
5
Is there any instrumentation on the market which can be fitted to the couplings?
Not to my knowledge. Sometimes the outer diameter
of a coupling is machined (see right side of picture on
slide 3 of the webinar) to allow torsional vibration
measurement with a speed pick-up.
What continuous condition monitoring actions can monitor torsional vibrations?
In torsional vibration condition monitoring, the main
interest lies in the lifetime consumption of the unit.
If events are detected that consume significant shaft
line lifetime, the power plant would be alerted. After
detection, more detailed analysis can be initiated to
understand the underlying cause and measures to
avoid the problem in the future.
To enable monitoring, a torsional vibration model
needs to be built based on the available technical
drawings. Simplified torsional vibration models
(required at the design stage) are typically provided
by the OEM. From a practical point of view, it needs
to be verified whether the existing speed sensors
can be used. In rare cases where this is not possible,
additional sensors need to be installed.
To identify the torsional frequencies and produce the models, is measurement data from one rotor sufficient (using speed probes or external measurements) or do you need to measure all the connected rotors separately?
The torsional vibration model, along with the mode
shapes, are typically available from the OEM. Based on
this information, the sensor location can be assessed.
Measurement at one location on the shaft line is
normally sufficient, at least for the subsynchronous
modes. It depends on the measurement location and
the mode shapes of the different torsional natural
frequencies. If the sensor is located in a node of
a mode, a single location is not enough. Typically,
2-3 sensors are sufficient to capture all the natural
frequencies.
6
How do you monitor torsional frequency on pumps and compressors?
The measurement principles are the same. As
explained in the webinar, there are many different
technologies available for measurement.
Can you say a bit more about how you managed to protect the unit from future torsional vibrations in the previous example?
The root cause analysis has unfortunately not yet
been finalized and so a final solution has also not been
implemented.
Would we be able to identify external interactions, such as load rejection or any grid transient, using phase difference between the turbine shaft and the generator shaft? Which area would most likely be affected from the perspective of visual inspections during outage?
Typically, the most important torsional vibration
modes are the subsynchronous ones. With one single
measurement on the shaft and the underlying model,
we can determine the stress levels at every point of
the shaft. Based on the mode shapes, we can already
identify (without measurement) the positions on the
shaft where damage is most likely to occur, which can
serve as inspection guidelines. Typically, this is on the
intermediate shaft between the main rotors at the
fillet transition of a coupling for instance.
What would you recommend as a good inspection regime for torsional vibration impact on rotors, or for transients?
This is difficult to answer as this would require
dedicated monitoring, such as with a torsional
vibration monitoring system. When such a system
is available, the fatigue lifetime consumption can
be tracked over time and serve as an input for
inspections.
Can the torsional vibration impact be mitigated at the mechanical design stage of the shaft?
Possible means of mitigation:
• eallocating the natural frequency if it is close to 2x
grid frequency;
• reducing the local stress concentration effect
(larger fillet radius for instance);
• if an intermediate shaft is present, ensure that this
is the weakest link, so that if torsional vibrations
damage the machine, you won’t have to replace a
rotor, which would be more costly;
• a torsional vibration damper.
In the examples, how would the retaining rings be affected?
Retaining rings are typically shrink-fitted onto the
generator rotor. If torsional vibrations are present,
the generator will oscillate back and forth. This
means that the retaining ring needs to follow this
oscillation as well. The inertia of the retaining ring
will induce a force onto the shrink-fit surface and
cause a phenomenon called fretting fatigue. This can
initiate micro-cracks on the surface which can further
propagate.
The energy transition in South Africa is a topical issue, and the 2019 integrated resource plan estimates that wind and solar will represent 23% and 11%, respectively, of electricity generation by 2030. With that in mind how can an existing wind farm adequately prepare itself against the risk of the uptake of renewables?
Torsional interaction has also been observed in wind
farms on occasion. In some incidents, the damage was
incurred within one second of interaction with a series
capacitor bank. There is increasing research ongoing
in order to improve understanding of this and risk
management.
Clearly, monitoring seems to be the first step in
gathering experience. In some cases, the electrical
parameters are already being measured and could
therefore be used as monitoring input.
Q & A Webinar 20 October 2020
7
Is the rupture more likely at high or low pressure?
Failure occurs at the weakest link in terms of fatigue
lifetime and depends on the frequency at which the
unit is excited. In general, these are locations where
the diameter is least. For high- and low-pressure
steam turbines, there is no general rule. For the lowest
vibration mode of a steam turbine unit, the highest
stress typically occurs between the generator and the
LP turbine.
Is it normal to have a problem every five years due to vibrations?
Vibration issues occur quite frequently, as is clear
from experience all over the world. This could be due
to design issues or changes in operational conditions
(e.g. grid configuration). There is no real time-based
relationship.
Does a normal FFT from a high quality timewave captured with an optical probe and zebra tape show the torsional vibration frequency?
Normally, the output is a digital signal (e.g. TTL)
which needs to be processed by TIM (Time Interval
Measurement). An FFT will generate a lot of
harmonics. The torsional frequencies will appear as
sideband modulations of the main speed harmonic
component. The amplitude, however, won’t make
much sense.
Are there any examples of ENGIE Laborelec’s torsional vibration system installed here in the UAE?
No. We have TORSO on 25 units in Peru and Chile
and six units in Europe. There are many different
temporary campaigns ongoing.
We experienced defects from variable frequency drivers (VFD). Can we set some variables to avoid these defects?
The first thing is to conduct a root cause analysis
to confirm that the VFD is causing the defects.
This requires at least analysis of the failures found
including metallurgical analysis and a design review.
A detailed measurement campaign (radial/torsional
vibrations, electrical measurements) can further help
to identify the issue (potential resonance?).
The VFD is known to be a source of torsional
vibrations (6x, 12x, 18x operating speed, and greater),
but other components on the train can also serve as a
source (e.g. pump impeller).
If the VFD is causing the issue, harmonic content could
be reduced by changing the type of VFD (e.g. Pulse
Width Modulation instead of Current Source Inverter).
If a resonance condition is identified, then the options
are to avoid the speed range at which resonance
occurs or to apply a structural modification in order
to reallocate the natural frequency outside of the
operating range.
8
Does variation in grid frequency cause the torsional vibrations? What are the typical values for the tolerable range of grid frequency? Are generator/turbine trips kept/based on these considerations?
Allowable grid frequency variation depends on the
grid standards. In Europe, the current ROCOF (Rate
of Change of Frequency) is rather low and in practice
it has never exceeded 1Hz/s. With the increasing
amount of renewables, this is expected to increase
and ROCOF values up to 2Hz/s should be anticipated.
The impact on the machine of an increased ROCOF
is on different levels. In a direct sense, the change
in frequency will cause an inertial torque on the
machine, the size of which depends on the actual
frequency gradient and the inertia of the shaft line.
Another impact is related with the control logics
(speed, voltage) at the power plant level. These should
be able to recover from such grid frequency changes
without rendering the system unstable.
The change in grid frequency itself has a limited effect,
unless for instance if torsional natural frequencies are
moved closer (too close) to 1x and 2x grid frequency.
In Europe the grid frequency is limited to 49.8Hz and
50.2Hz.
Any known failure examples for hydropower units?
Not to my knowledge. For hydro turbines, the risk for
grid interaction is typically low because the generator
has a very large inertia compared to the turbine.
It therefore acts as a filter, effectively blocking
oscillating energy from the grid. The water from the
turbine also provides more damping to the torsional
vibrations compared to GT and ST units where the
overall damping is very low (only material damping
and small damping due to steam at larger LP blades).
Q & A Webinar 20 October 2020
9
10
The TSO usually provides the harmonics background at the connexion point. Could this information be used to assess potential transfer to turbines?
The harmonics play a role in the power quality and
are therefore an indication of perturbations on the
grid. For instance, they can be used as an indication
for what interaction with an AC arc furnace plant
involves. If a large amount of harmonic energy
content is present, the impact on the conventional
power plant might be significant. However, interaction
occurs typically at subsynchronous frequencies which
is not reflected by the harmonics. The useful info they
can provide is therefore limited.
In our power plant there is a GE torsional protection system based on the speed signal of ST. Is that speed signal enough to protect the machine?
For protection, more than one sensor should be used
for redundancy. Whether or not one axial location on
the shaft line (with two sensors) is enough depends
on the mode shapes. If that sensor is able to see the
different mode shapes, then this can be sufficient.
How much has the interaction of negative sequence current components in torsional vibrations been considered? It is known that during line-to-line short circuits, high magnitudes of this current occur and during unbalanced loads. Is this component of the stream considered within TORSO?
Negative sequence currents introduce excitation at 2x
grid frequency. At the design stage, exclusion zones
are imposed for natural frequencies near 1x and 2x
grid frequency. Some guidelines are given in the ISO
22266-1 standard. At 100Hz, the theoretical model is
more prone to errors, such that these exclusion zones
are not met. Different LP blade failures have been
reported in the past. When measuring on-site, the
exclusion zones are verified (with TORSO for instance)
and mitigation actions can be taken (see for instance
the first example in the webinar) if needed.
Is there a technology to disconnect the grid breaker to isolate the source of torsional vibration excitation?
Yes, there are several so-called torsional vibration
protection systems on the market, typically offered
by OEMs. Laborelec has developed its own protection
system, which is currently installed on more than 30
units. The main functioning is based on the underlying
fatigue damage consumption as described in the
webinar (slide 30). When significant fatigue damage
is being consumed, the protection system will trigger
a relay which will automatically trip the unit and
disconnect it from the grid.
Q & A Webinar 20 October 2020
11
For GT unit maintenance, what could be the recommended "standard" inspection to be carried out using PT (penetrant testing) on the shaft, bearing in mind that they are units that have daily starts?
Based on the model and technical drawings, the most
critical locations can be identified for inspection. This
will be on sections where the diameter is smallest and
where the stress concentration is highest, e.g. at a
fillet radius of a coupling for instance.
How can you identify a compressor crack in a vibration analysis?
A crack is not always visible in the radial vibrations.
Typically, it is only identified when they have reached
a considerable size compared to the rotor diameter,
such that the local unbalance and/or stiffness change.
It can then be seen in the 1x and/or 2x components of
the radial vibrations, either during the transient at the
critical speed or at nominal speed.
The increase in vibration amplitudes can be over the
course of less than a day or over several months. This
depends on the crack location, geometry, length or
other factors. There is no real generic rule.
What is the amplitude of frequency (+/- 50 Hz) on the grid to avoid torsional vibration?
Variation of grid frequency in itself does not
cause torsional vibrations, unless natural torsional
frequencies are moved closer (too close) to 1x and 2x
grid frequency or where control interaction occurs
after these grid frequency changes.
Can the rotating stall of a compressor cause torsional vibrations?
A phenomenon has been reported where the rotating
stall can couple with the torsional mode of the
blade dynamics. To further couple with the torsional
vibrations of the entire shaft line, the zero nodal
diameter of that blade row would need to be excited
and the torsional natural frequency should lie at the
same frequency. I would not exclude it a priori, but
the chances are slight.
How do steam flow variations influence torsional vibrations?
Steam flow provides some excitation of the shaft line,
but this is rather random, and the resulting torsional
vibration response is limited.
Can the phenomena originate from the power train control system, such as GT gas valve speed or main steam turbine valve speed?
Yes, in addition to excitation due to grid interaction,
excitation is also possible due to internal control
instability. Examples are voltage control instabilities
due to interaction with the Power System Stabilizer
(PSS). Speed control instability has also been reported
due to a malfunctioning gas fuel valve. Any control
action which can interfere at higher frequencies can
introduce torsional vibration issues. In most cases, a
torsional filter can be used to resolve the instability
issue.
12
We have suffered some cases where gas compression failures have caused broken engine crankshafts. Do you have any experience of compression equipment you can share?
Not directly. In Belgium we have encountered a failed
crankshaft on a diesel engine where propagation was
due to torsional vibrations initiated by excessive
bending forces. It would be interesting to know
whether a root cause analysis has been performed on
your cases.
How many locations do you need to measure?
In most cases, one axial location on the shaft line is
sufficient. Sometimes a second temporary sensor
is added in order to have some validation of the
underlying model.
Does the position of your sensor influence the quality of your calculations?
If the sensor is too close to a nodal point of the
mode shape, the uncertainty can become too great,
and significant errors could be introduced in the
correlation between measured torsional vibration
amplitude and stress level elsewhere.
Q & A Webinar 20 October 2020
13
Can a torsional vibration expert detect torsional vibrations using conventional vibration monitoring systems (with sensors installed on the bearings measuring radial and axial vibration and a tacho), such as System I?
Except for shaft lines with a gearbox, there is no
strong coupling between the radial vibrations and
the torsional vibrations. The radial vibrations cannot
therefore typically be used to assess torsional
vibrations.
The tacho typically aims at providing an average
speed signal in which the higher torsional frequency
content is not present. The tacho might be used
(depending on what type of signal comes out), but
dedicated hardware/software is needed to convert
this to torsional vibrations.
Is torsional vibration cumulative? To assess the risk, is it necessary to quantify the cumulative damage and remaining life?
The underlying damage mechanism is fatigue. Fatigue
damage accumulates over time and so, to assess the
risk of continued operation, the remaining lifetime
needs to be determined. We have applied this in
the past to assess the risk of continued operation of
parallel units after the failure of one or more units of
the same power plant.
Do cracks in the shaft due to torsional vibrations start on the outside or the inside of the shaft?
They start from the outside because that is
where stress levels are highest (linear increase
with diameter). Initiation is typically at stress
concentrations (e.g. fillet).
14
Is there a difference between torsional vibration damage on generators and on electromotors?
On large synchronous generators, damage observed
at the generator due to torsional vibrations is at the
retaining rings, which fail due to fretting fatigue. For
electromotors, it’s a completely different story — the
entire drive train needs to be considered. Damage is
more likely to occur in the intermediate parts (e.g. at
couplings).
How accurate is using a magnetic pick-up compared to an encoder used continuously?
The encoder also provides a pulse train. The difference
with a magnetic pick-up is that the tooth profile is
more uniform compared to a machined toothed wheel.
The number of teeth is also typically higher than for a
toothed wheel but, as we are interested in frequencies
below 200Hz, this does not have an impact on
accuracy.
Q & A Webinar 20 October 2020
15
Among the potential harmful conditions for turbo-
groups, torsional vibration is one that is generally
overlooked. Because it can lead extremely quickly to
dramatic failures, effective protection systems must
be able to react within seconds.
TORSO, Laborelec’s torsional vibration monitoring
system for turbo-groups, tackles this issue
effectively and brings peace of mind back
to your condition monitoring strategy.
An understated but real enemy
Although the risk of damage due to torsional
vibration is generally well assessed at the
design stage of a power generation system, later
adjustments to the shaft line, or modifications to
the external power system, can create the perfect
conditions for certain phenomena to develop. These
conditions include sub-synchronous resonance or
interactions with high voltage DC stations which, in
turn, lead to excessive levels of torsional vibration.
In most operational environments, this specific
type of vibration is not tracked, even though it
can abruptly lead to heavy damage such as shaft
cracking, blade loss or gearbox failure.
TORSO keeps the installation safe
Using specific sensors, easily mountable on toothed
wheels or on shaft ends, TORSO protects the shaft
line from the effects of harmful torsional vibration.
It rapidly detects when this reaches pre-defined
thresholds, and trips the unit before severe
damage can occur.
What is torsional vibration?
Torsional vibration is angular vibration of
an object—commonly a shaft along its axis of
rotation. It is superposed to the static torsion
and occurs at the frequency of the torsional
excitation. It may coincide with the shaft's
natural frequencies. Torsional vibration
is often a concern in power transmission
systems using rotating shafts or couplings
where it can cause failures if not controlled.
TORSO, ENGIE Laborelec’s torsionalvibration monitoring and protection system Keep a watchful eye for harmful torsional vibration in shaft lines
Typically, between 1 and 3 well-positioned sensors are
sufficient for the system to offer full protection. Our experts
can advise on the most appropriate sensor locations, since
these are machine-specific. In addition, existing speed sensors
can be used in most cases.
Mounting of a sensor at the turbine - generator coupling
16
Torso
17
Benefits• EASY TO INSTALL. Adjustable and easy
to install on the most commonly-
encountered types of machines.
• OPTIMISED ALARM STRATEGY. Unique, intelligent strategy for triggering
alarms and tripping.
• MONITOR & DIAGNOSE. Historical data
available for further analysis and
troubleshooting.
• NOT A BLACK BOX. Open system allowing
power plant staff to visualise and work
with data.
• VERSATILE. Available for both
permanent and temporary monitoring.
18
How it works
Each specific machine is different and vibrates in a slightly
dissimilar way. Critical locations where material damage is
most likely to occur are precisely calculated due to TORSO’s
underlying theoretical shaft line modelling.
The need for modelling
Torso
Initial modelling of the shaft line
• A specific rotor mode shape analysis makes
it possible to select the best locations for the
sensors in accordance with the expected
mode shapes.
• The results of this initial modelling is validated
at a later stage with on-site measurements,
to ensure the best protection level
is achieved.
Installation and validation
• Sensors are installed at different locations along
the rotor to build in redundancy and provide
better signal processing functionality for
the system.
• Alarm or trip signals can be wired either to the
control system or to one of the generator relays.
Frequency and amplitude signals of monitored
frequency bands can be wired to the DCS or
SCADA system.
Monitoring
• Frequency and peak amplitude of critical
resonance frequency bands are continuously
monitored by the system.
• The common alarm/trip strategy comprises a
watchdog alarm, an event detection level alarm
and a trip level alarm.
Diagnosis of specific issues
• Through its customisable interface, measurement
data are presented for analysis, diagnosis and
identification of remedial action by power plant
staff or Laborelec experts.
Alarm thresholds set too high will lead to undetected
excessive torsional vibration and severe damage.
However, excessively low thresholds can generate
a large number of unwanted alarms. The model behind
TORSO makes it possible to calculate precise thresholds,
and determine a safe middle course.
Finding the right alarm thresholds
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